1. Field of the Invention
The present invention relates to an electrode device and apparatus for generating plasma, and more particularly, to an electrode device and an apparatus for generating plasma in an upper portion of a substrate disposed on a susceptor according to a high frequency signal supplied to the electrode device.
2. Description of the Related Art
In semiconductor and display fabrication processes, plasma technology has been more important and plasma has been widely used, because plasma is gas comprised of charge particles and electrons, ions and active species in the plasma can be controlled by controlling parameters such as an electric field, a magnetic field, etc.
In the current fields of semiconductors and displays, a method of generating plasma by using a radio frequency has been mostly used. The method of generating plasma by using the radio frequency includes a method of generating plasma by using inductively coupled plasma (ICP) and a method of generating plasma by using capacitively coupled plasma (CCP).
Structural characteristics of the method of generating plasma by using the CCP are in that there is an electrode device designed to apply voltage. As known from the name of CCP, plasma is generated and sustained by a capacitive electric field formed due to charges distributed on the surface of the electrode device. In the method of generating plasma by using the CCP, plasma is generated and controlled by an electric field formed between upper and lower electrodes. The electric field is very sensitive to shapes of the upper and lower electrodes and material properties, mechanical characteristics, electrical characteristics and geometrical characteristics of peripheral structures. Thus, it is difficult to control the plasma by using the electric field. Thus, considerable know-how is needed to constitute hardware.
As described above, plasma is used to form a layer by using a predetermined material on a large-sized substrate in the fields of semiconductors, displays, and solar light. In this case, in order to attain productivity with low costs and high efficiency, the large-sized substrate needs to be processed. In order to process the large-sized substrate, high-density plasma having high deposition speed should be generated. High electrical energy is used to generate the high-density plasma, and a high supply frequency is needed to improve energy transmission between the supplied electrical energy and plasma.
In the related art, a large-sized flat electrode device is used to form a layer by using a predetermined material on a substrate by using plasma. FIG. 1 is a view of a large-sized flat electrode device disposed on a susceptor, and FIGS. 2A through 2F are images showing losses of electrical power that occurs in the large-sized flat electrode device of FIG. 1 according to frequencies. Referring to FIGS. 2A through 2F, frequency is improved so as to generate a high-density plasma. As frequency increases, a rapid loss of electrical power occurs in the large-sized flat electrode device of FIG. 1.
In order to solve the problem of the large-sized flat electrode device of FIG. 1, a linear electrode device including linear elements has been developed. Plasma having the frequency of 30 MHz to 300 MHz at a very high frequency (VHF) band is required to reduce ion collision energy and to perform high-efficiency deposition. FIG. 3 is a view of a conventional ladder-shaped electrode device. Referring to FIG. 3, electrical power having the frequency of 60 MHz at a VHF band is divided into two routes and is supplied to one and the other ends of the electrode device. In this case, electrical power having the same frequency having only a phase difference caused by a phase shifter is supplied to one and the other ends of the electrode device.
FIGS. 4A and 4B are images showing the distribution of electric potentials formed on the surface of the conventional ladder-shaped electrode device illustrated in FIG. 3 when electrical power having the same frequency is supplied to the ladder-shaped electrode device of FIG. 3. Referring to FIGS. 4A and 4B, when electrical power having the frequency that is equal to or lower than 40 MHz is supplied to the ladder electrode device of FIG. 3, a forwarded wave is generated and uniform plasma is generated. However, when electrical power having the frequency that is equal to or higher than 60 MHz is supplied to the ladder electrode device of FIG. 3, distribution of electric potentials formed on the surface of the electrode device causes a pattern of a standing wave, and plasma corresponding to the pattern of the standing wave is generated. In this case, a node that is a characteristic of the standing wave is formed so that uniformity of the plasma lowers.
As described above, frequency at a VHF band is used in discharge so that high-density plasma causing a strong chemical reaction with a large-sized substrate can be generated. The density of the generated plasma is in proportion to the square of a discharge frequency. As the thickness of a boundary surface of the plasma decreases due to high resonant characteristics at the VHF band, energy coupling between the ladder-shaped electrode device of FIG. 3 and the plasma is further improved and the density of the plasma is increased. However, a reduction in wavelength according to an increase in plasma discharge frequency causes variation of amplitudes of voltage and current inside a source and variation of phases so that strong wave characteristics of voltage and current occur. When the standing wave that is one of the wave characteristics oscillates on the electrode device, the uniformity of a surface on which the plasma is to be deposited lowers greatly. Also, due to the formation of a node that is a characteristic of the standing wave, the density of the plasma at the surface on which the plasma is to be deposited lowers. Thus, a degree of ionization of neutral particles lowers, and discharge is not partially performed so that deposition cannot be properly performed. Accordingly, an electrode device that prevents generation of the standing wave is needed.